Some of these may be common sense, but I find myself or others falling into these pitfalls quite commonly. Try to avoid them!

If you are low on time with an absolutely dominating position (like the player who lost on time with a queen and rook vs a pawn), just try to make sure that the other player will have no mating material. Even if you don't have the time to mate, you can at least make sure you have a draw in hand.

The object of the game is to win. If you see a sacrifice that wins instantly, go for it. However, unless your name is Mikhail Tal (And I assure you, it isn't), it is generally best to check for a better move after you consider an unclear sacrifice. If there isn't, go for it. However, if you have the time, you are more likely to earn a clean win than a messy one where you might lose.

If you fall into a slump, take a break. Think and relax. Then come back later. You will be less likely to be over-aggressive when you try to get your rating back.

If you find yourself getting emotionally attached to your play, also take a break. You want to be level-headed, or you might play too aggressively again.

Don't play late into the night. When you're tired, you're less likely to play sound chess.

If your opponent is low on time, a good way to try to pressure the clock may be to throw in some unexpected moves or checks. The unexpected check or rook sacrifice you may play might throw them off long enough to salvage a draw or win a drawn position or a time scramble. Also, you can in some time controls where you don't add time to the clock, if your opponent is low on time, repeating moves and holding the position can make your opponent rush and make hasty moves to make your job easier (assuming the position allows for this).

If low on time in a time control setting where you can add time to your clock by moving, it can be a good strategy to force a repetition of moves when you can. You can earn more thinking time for yourself and allow yourself to calm down and take full control of the situation at hand.

Premove recaptures or if your opponent has to make a really obvious move (obvious recapture, move out of check, etc.) Don't premove when your opponent has too many options, however, or he could catch you off guard and this would hurt.

All great advice. I would add that on Chess.com, and maybe other sites, when you're playing bullet/blitz chess you can drag a piece over the square you want to move it to while it's still your opponent's turn. When he moves, just let go and you've made your move in about 0.1 seconds. The only problem is that you better be sure the move will be a good one because you're not giving yourself much time to think it over once it's your turn. It works well when your opponent's move is forced and you're sure of what the proper response will be.

If your opponent is low on time, a good way to try to pressure the clock may be to throw in some unexpected moves or checks. The unexpected check or rook sacrifice you may play might throw them off long enough to salvage a draw or win a drawn position or a time scramble.

There are tons and tons of time wasting moves. The main idea is to hold a good position with a slight variation. And if you see that your opponents time is less than 1/4th of yours, always hold your 'set' position. Let him make hasty moves and blunder. Triangulation is something I use the most.